Conventional turbomolecular vacuum pumps generally comprise an external housing, which houses the gas pumping stages, obtained through the co-operation of stator rings integral with the pump body and rotor disks integral with a rotating shaft rotated by the pump motor.
In many applications, pumps with the smallest possible size, in particular in axial direction, are required. Analytical instruments, such as mass spectrometers or gas chromatographs give an example of such applications.
Several compact turbomolecular vacuum pumps have already been proposed in the past.
U.S. Pat. No. 5,542,825 discloses a turbomolecular vacuum pump with a bell-shaped rotor, i.e. a rotor having an axial cavity extending over part of the rotor length. The corresponding stator has a portion external to the cavity and provided with stator disks co-operating with rotor disks for forming the pumping stages, and an internal portion penetrating into the cavity and supporting the stator portion of the motor. The motor rotor is a permanent magnet placed in a central bore of a shaft forming an integral with the pump motor.
U.S. Pat. No. 6,158,986 discloses a pump with a bell-shaped rotor, the cavity of which houses the pump motor, which is an “inverted” motor, that is a motor with internal stator and external rotor. The motor stator being an integral with the pump body and the motor rotor is coupled with the internal surface of the cavity.
U.S. Pat. No. 6,179,953 discloses another example of inverted-motor pump, in which the motor rotor axially extends over substantially the whole internal cavity of the pump housing and forms also the pump rotor.
Both the solution using a motor with conventional arrangement, and the solutions using an inverted motor have some drawbacks: the pump rotor is heavy and gives rise to balancing problems; moreover, rotating magnetic parts exist. For the vacuum pump disclosed in the U.S. Pat. No. 5,542,825 a modified motor is especially needed, with a rotor inside the shaft, however the structure would be complex.
Thus, it is an advantage of the present invention, which provides a compact vacuum pump, with high structural simplicity.
It is another object of the invention to provide a compact turbomolecular vacuum pump, which is easy and inexpensive to manufacture.